All active noise cancellation products, whether they are based on a feedback or feedforward control architecture (or a combination of these two architectures) require a tailored transfer function between the noise sensing device (typically one or more sensing microphones) and the device that creates the acoustic response required to cancel the sensed noise (typically a speaker). In this document the transfer function between the sensing microphone (s) and the speaker is referred to as the control law transfer function. This transfer function facilitates the realisation of noise cancellation over a suitable bandwidth whilst minimising noise amplification and/or instability outside this bandwidth.
Classically the control law transfer function has been realised through use of an analog filter which consists of a fixed combination of active and passive components. Such a realisation has the following disadvantages:                1. Its fixed nature does not allow adjustment of production variation within the electro-acoustics to which it interfaces.        2. its fixed nature does not allow easy accommodation of a range of electro-acoustic designs.        3. Its fixed nature does not permit dynamic adjustment of the control law to provide optimised noise cancellation based on the prevailing noise field.        4. The component count of the implementation is high and typically does not lend itself to integration within an integrated circuit.        